Sepsis is a significant healthcare issue due to its high frequency of occurrence and high mortality rate in hospitals. Sepsis is characterized by a whole-body inflammatory state, called a systemic inflammatory response (SIRS), and by the presence of a known or suspected infection. The immune system may cause this inflammatory response as a consequence of microbes in the blood, urine, lungs, skin, or other tissues, for example. One of the leading causes of sepsis is a bloodstream infection (BSI). BSI is most commonly diagnosed by a blood culture, in which a sample of blood is incubated with a medium in an atmosphere controlled to promote bacterial growth. Current automated blood culture systems can take 12-48 hours to detect the presence of infectious microorganisms in blood and can take up to 5 days to rule out the presence of any infectious microorganisms. It can take up to another 12-48 hours to identify the infectious microorganisms by sub-culturing the positive blood culture and performing identification and antimicrobial susceptibility tests. These results can be too late to alter the treatment course and result in the death of the patient. It would be advantageous if the time it takes to detect the presence of infectious microorganisms in the blood or other body fluid or tissue could be shortened to less than 24 hours, and more preferentially to less than 8 hours. Consequently, more time effective methods and apparatus for detecting the presence or absence of infectious microorganisms in a biological sample to determine, for example, if a patient has a BSI continue to be sought.
Bacteria in clinical blood samples are usually detected by inoculating approximately 10 ml of whole blood in a culture bottle, incubating the bottle in an automated system at 35° C., and monitoring products of bacterial metabolism (such as carbon dioxide) by means of sensors disposed within the culture bottle.
The presence of a growing bacterial population within a culture bottle of 90 ml overall volume is typically detected when the number of microorganisms has risen to approximately 5×109. Many bacterial doubling events are required to grow a bacterial population from one or two organisms in the 10 mL blood sample to such a high number. One approach to faster bacterial detection is splitting the 10 ml sample liquid together with the required growth media of typically 40 mL volume into a large number of smaller partial samples that are contained in closed small chambers (see U.S. Pat. Nos. 5,770,440 and 5,891,739 to Berndt, the entire contents of which are both hereby incorporated by reference herein). If the small chambers are not closed, but have a joint head space volume, the shortened Time to Detection (TTD) that is achieved may be less than is desired (see U.S. Pat. No. 5,716,798 to Monthony et al., the entire contents of which are hereby incorporated by reference herein).
While the splitting of the original 10-mL blood sample together with the 40 mL of growth media may achieve faster bacterial detection, the design of a practical multi-chamber sample container remains challenging. Also, if one or two of the small chambers contain sample that shows signs of bacterial growth, there is a need for removal of the sample liquid from those chambers for post-processing procedures such as identification or antibiotic susceptibility testing. Such sample removal represents a further challenge. Also, it is not certain that 10 mL of clinical blood sample is such a small volume that it will contain only one colony forming unit (“CFU”). Such small volumes may very well contain not only two but maybe up to 100 CFUs. In this case, one would waste valuable detection time by distributing the organisms of such bacteria-rich sample into many chambers, whereby each chamber would likely contain either no organism or one organism. For each individual organism, it would take approximately seven doublings to achieve again the number 100. Seven doubling times of approximately two hours for slow growers would mean 14 hours of lost detection time.
In view of this, there exists still a need for a faster bacterial detection technique that neither (i) requires a multi-chamber sample container nor (ii) is prone to wasting valuable detection time in the case of bacteria-rich blood samples.